The present invention is in the field of ceramic glazing and particularly relates to a method for spray-decorating low-expansion glass, glass-ceramic or ceramic ware with enamels having relatively high thermal expansion coefficients.
As is well known, typical enameling compositions for decorating glass or ceramic substrates comprise a pigment component and a fluxing component. The pigment consists of a highly colored inorganic element or compound, which may or may not be admixed or encapsulated with a glass or other matrix material, while the flux typically consists of a fritted, low-melting glass, i.e., a glass in finely divided form. The fluxing component, which frequently comprises at least about 80% by weight of the enameling composition, fuses when the enamel is fired to provide an impermeable, chemically durable matrix for the pigment.
One of the problems presented by the development of low-expansion ceramic products such as glass-ceramic culinary ware has been the development of enameling compositions exhibiting sufficiently low thermal expansion for these products. A suitable enamel or glaze will have a thermal expansion coefficient on the same order of, or preferably below, the expansion coefficient of the ceramic body to which it is applied, so that the enamel will not crack or craze in use. It is difficult to formulate an enamel flux glass which fires to a smooth enamel yet has an expansion coefficient sufficiently low to avoid crazing or spalling from a low-expansion substrate, because glass constituents which aid fluxing frequently increase the expansion coefficient of the enamel.
U.S. Pat. No. 3,663,244 describes a family of enameling compositions which devitrify on firing and thus provide an enamel layer having a relatively low expansion coefficient, e.g., an average linear coefficient of thermal expansion (expansion coefficient) in the range of about 15-30.times.10.sup.-7 /.degree.C. These expansion coefficients are sufficiently low to make the enamel compatible with low-expansion glass, glass-ceramic or ceramic articles, which for the purpose of the present description are articles having expansion coefficients in the range of about 0-35.times.10.sup.-7 /.degree.C.
Enameling compositions such as disclosed in the aforementioned patent may be applied to large areas of a ceramic substrate by spraying. For this purpose, an aqueous fluid suspension of the composition, having a viscosity suitable for spraying (typically in the range of 10-200 poises), is prepared. This suspension includes a water base, an alcohol component for rapid drying, and the fritted enameling composition, and is applied in a thickness which will provide a fired enamel layer having a thickness in the range of 1-2 mils (25-50 microns) on the glass-ceramic surface to be decorated. This thickness provides adequate gloss and color in the glazed article.
Enameling compositions offering more intense colors and good chemical durability and gloss when applied at somewhat lower firing temperatures have been developed for application to opal glassware, as described in U.S. Pat. Nos. 4,038,448 and 4,158,080. A related family of enameling compositions, described in U.S. Pat. No. 4,158,081, has been proposed for lower expansion glass or glass-ceramic articles, ie., articles having expansion coefficients below 40.times.10.sup.-7 /.degree.C. The compositions described in these patents provide enamels hereinafter referred to as high-expansion decorating enamels, having expansion coefficients after firing which are typically in the range of 45-75.times.10.sup.-7 /.degree.C. This is substantially higher than the expansion coefficients of the low-expansion devitrifiable enamels referred to above, and higher than the expansions of some of the low-expansion ware to which these enamels would desirably be applied.
High-expansion, low-firing enameling compositions such as described in these last mentioned three patents are typically applied as very thin films to glass or glass-ceramic substrates. Application is by silk-screening, or by transfer from decals which are prepared by silk-screen or lithographic techniques. The compositions comprise the conventional fritted glass flux and pigment components, but they are applied as constituents of a screening paste. The screening paste is typically a non-aqueous medium comprising a silk-screening oil base and a volatile solvent in combination with the enameling composition, the oil and solvent constituting the vehicle for the fritted enamel.
It would be desirable to be able to apply large-area coatings of these newer, high-expansion, low-firing enamels to low-expansion glass, glass-ceramic or ceramic ware by spraying, instead of by decals or screening. The new enamels exhibit higher gloss and better color characteristics than the older devitrifiable enamels, and provide excellent chemical durability. However, attempts to apply such enamels by spraying have not been successful. Substantial difficulties have been encountered in trying to obtain good coverage and uniform coloration in the sprayed layer, while at the time achieving a thin fired enamel layer which will not craze or spall from the low-expansion ceramic substrate in use. Excessively thick enamel layers do not adhere to lower expansion substrates because of the relatively large thermal expansion mismatch between the enamel and the substrate.
It is therefore a principal object of the present invention to provide a method for applying a high-expansion enameling composition to a low-expansion glass, glass-ceramic or ceramic article which produces a glossy and uniformly colored enamel layer thereon.
Other objects and advantages of the invention will become apparent from the following description thereof.